Minimum viable population

The smallest population size estimated to have a specified probability of persistence over a defined period. It depends on the species, threats, habitat, and chosen time horizon.

Population viability analysis: A quantitative method for estimating a population’s risk of extinction over time from demographic, environmental, and genetic data. It is the main modeling framework used to estimate minimum viable population size.
Extinction risk: The probability that a species or population will cease to exist within a specified period. Minimum viable population is defined by an acceptable extinction-risk threshold.
Species recovery plan: A coordinated plan for restoring a threatened species and reducing its risk of extinction. Population targets in recovery plans may be informed by viability estimates.
50/500 rule: A historical rule of thumb proposing effective population sizes of 50 to limit inbreeding and 500 to retain evolutionary potential. Unlike a tailored viability estimate, it gives fixed genetic guidelines rather than a probability over a defined period.
Demographic stochasticity: Random variation in births, deaths, and sex ratios among individuals in a population. Chance fluctuations in vital rates can threaten small populations even in stable environments.
Effective population size: The size of an idealized population experiencing genetic drift at the same rate as a real population. It can be much smaller than the census count relevant to a viability estimate.
Population reinforcement: The addition of individuals to an existing population to improve its survival prospects. Reinforcement can raise population size or genetic diversity toward a viability target.
Extinction vortex: A feedback process in which small population size worsens genetic and demographic decline, further shrinking the population. It describes a self-amplifying decline, not a numerical persistence threshold.
Environmental stochasticity: Random changes in environmental conditions that affect survival or reproduction across a population. Bad years can reduce a population regardless of its initial size.
Census population size: The number of individuals in a population at a specified time. It is the count commonly compared with an estimated minimum viable population.

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